Propagate Sync error when running SafeFileRotate (elastic#9069)

Previously, it was possible SafeFileRotate encountered an error without propagating it, because the return value of os.Sync was not utilized.
From now on the errors from Sync are propagated.

CHANGELOG.asciidoc

@@ -54,6 +54,7 @@ https://github.com/elastic/beats/compare/v6.4.0...master[Check the HEAD diff]
 - Fixed Support `add_docker_metadata` in Windows by identifying systems' path separator. {issue}7797[7797]
 - Do not panic when no tokenizer string is configured for a dissect processor. {issue}8895[8895]
 - Start autodiscover consumers before producers. {pull}7926[7926]
+- Propagate Sync error when running SafeFileRotate. {pull}9069[9069]
 
 *Auditbeat*
 

libbeat/common/file/helper_other.go

@@ -40,7 +40,6 @@ func SafeFileRotate(path, tempfile string) error {
 		return nil // ignore error
 	}
 	defer f.Close()
-	f.Sync()
 
-	return nil
+	return f.Sync()
 }